(a) Field of the Invention
The present invention relates to a compact zoom lens system with a high zooming ratio comprising three lens groups.
(b) Description of the Prior Art
Recently, with the development of automation in camera systems, a taking lens system of a lens shutter camera which is characterized by being very easily carried has also started to become equipped with a zoom lens system. Therefore, it has been necessary to develop a compact zoom lens system.
This kind of lens system mainly installed on lens shutter cameras does not have a limitation within which the predetermined back focal length should be maintained as in the case of a lens system of single-lens reflex cameras, so that it is possible to make a lens system compact by means of shortening the back focal length.
It has been known in theory that, in order to make a lens system compact by means of shortening the back focal length, it should be effective to adopt a configuration comprising two lens groups the refracting power distribution of which is, in order from the object side, positive and negative. Compacting of, for example, a semi-wide-angle single focal lens system for a lens shutter camera and a varifocal lens system of which focal length is changeable into two states has been made possible by the above-mentioned means.
In order to make a zoom lens system small, there has been also known the zooming system comprising two lens groups having the refracting power distribution mentioned as above. From the development of this zooming system there has been also suggested a zooming system comprising three lens groups, and been proposed a number of zoom lens system for a lens shutter camera. However, the zooming ratios of these conventional zoom lens systems are about 1.5, which does not satisfy the needs of the general users who desire changes of perspective in their photographs. In order to satisfy these needs of users, it is desired for a zoom lens system to have a zooming ratio of more than 2 or at least 2. Besides, the users of lens shutter cameras have had dissatisfaction with the optical performance thereof so that it is also necessary to improve optical performance.
As a zoom lens system of this kind, there has been known the zoom lens system of two groups-type which comprises, in order from the object side, a first lens group having a positive refracting power and a second lens group having a negative refracting power as disclosed in Japanese Published Unexamined Patent Application No. 201213/82, etc. The zoom lens system having a simple lens configuration of so-called three groups-type zoom lens system in which a portion of the first or the second lens group of the two groups-type zoom lens system is divided has been disclosed in Japanese Published Unexamined Patent Application No. 137813/83.
These zoom lens systems are intended to be used in lens shutter cameras, video cameras or the like and have no necessity of consideration for the limitation of a back focal length unlike in the case of single-lens reflex cameras, so that the compactness thereof has been attained by the shortening of the back focal length.
However, in view of the compactness of a lens system and the occurrence of flare due to harmful rays etc., it is necessary also for a lens shutter camera to avoid an excess shortening of the back focal length caused by attention only to the correction of aberrations.
Moreover, despite the zooming ratio being only about 1.5, there are many lens systems in which optical performance is not sufficient, especially, the corrections of chromatic aberration and curvature of field are so deficient that there is no room for windening the zooming ratio.
In this regard, in the zoom lens system of two groups-type or that of three groups-type developed therefrom, the attempt to attain the high zooming ratio causes an increase in the maximum movement due to the zooming of the lens groups which have the role of varying the focal length and an increase in the load for magnification, so that it is difficult to attain a sufficient optical performance for the entire zooming range.
In view of the above-mentioned points, the compactness of a zoom lens system is attained by the zooming system comprising four lens groups, as shown in FIG. 1 of which refracting powers are positive, negative, positive and negative in order from the object side, namely, comprising a first lens group G.sub.1 having a positive refracting power, a second lens group G.sub.2 having a negative refracting power, a third lens group G.sub.3 having a positive refracting power and a fourth lens group G.sub.4 having a negative refracting power each group of which moves as shown in FIG. 1 during the zooming operation and each of which takes a share of the overall magnification.
There has been also known the zoom lens system comprising four lens groups where, in order to simplify the structure of the lens-barrel, the configuration of the lens groups and the movement thereof are such as shown in FIG. 2. This zooming system has been proposed in Japanese Published Unexamined Patent Application No. 57814/85 and has the characteristic that the second lens group G.sub.2 is fixed during the zooming operation so that only the other three lens groups move. But, in the case where the zoom lens system of this type is intended to have larger magnification, the second lens group is fixed so that, in order to avoid the mechanical interference of the lens groups, it is necessary to suffciently enlarge the space on the optical axis between the second lens group and the third lens group in the wide-angle position, which makes it impossible to maintain the compactness thereof.